The short answer is, as far as I know, that it can't be done.
The longer answer is you would have to create your own widget to get a spinner that could have multiple selections in it, possibly by putting a multi-select ListView into a dialog then add a button for when the user is done selecting items. But this technically is not giving you "A Spinner control with multiple select option" because it's no longer a Spinner at all.
Even if you wanted to do this there would be some significant design decisions to make. For starters, how would your Spinner show you what's been selected on a single line like the Spinner does now with one item? Is this even important in your application? I'm not saying you can't do it, but it's not trivial and would really take some forethought on user interaction before even thinking about implementation.